One standard gas system used in AR-15 rifles is commonly referred to as Direct Impingement (DI). When a cartridge is fired, a portion of the expanding propellant gas is directed out of a gas port in the barrel through a gas tube and back into the upper receiver. As the gas then enters the bolt carrier through a gas key the bolt carrier is forced to the rear, unlocking the bolt. Thus, the cycling process begins, with the bolt ejecting the spent cartridge in preparation for acceptance of another shell.
Gases released through the bolt carrier serve to offset recoil and other impact forces in the firearm, subject to barrel length and bolt carrier weight. Additional variations due to ammunition will affect performance as a given amount of gas is directed back into the bolt carrier. Performance of the firearm in the ejection of spent shells may be tuned by controlling the amount of gas in the loop. A variety of solutions exist for regulating the amount of gas used in the system, most of which include an adjustable gas block on the barrel.